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<div class="titlepage"><div><div>
<h2 class="title">
<a id="_search_options"></a>Search Options<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch-definitive-guide/edit/2.x/060_Distributed_Search/15_Search_options.asciidoc">edit</a>
</h2>
</div></div></div>
<p>A few optional query-string parameters can influence the search process.</p>
<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="_preference"></a>preference<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch-definitive-guide/edit/2.x/060_Distributed_Search/15_Search_options.asciidoc">edit</a>
</h3>
</div></div></div>
<p>The <code class="literal">preference</code> parameter allows you to control which shards or nodes are
used to handle the search request. It accepts values such as <code class="literal">_primary</code>,
<code class="literal">_primary_first</code>, <code class="literal">_local</code>, <code class="literal">_only_node:xyz</code>, <code class="literal">_prefer_node:xyz</code>, and
<code class="literal">_shards:2,3</code>, which are explained in detail on the
<a href="/guide/en/elasticsearch/reference/2.4/search-request-preference.html" class="ulink" target="_top">search <code class="literal">preference</code></a>
documentation page.</p>
<p>However, the most generally useful value is some arbitrary string, to avoid
the <em>bouncing results</em> problem.</p>
<div class="sidebar">
<a id="bouncing-results"></a>
<div class="titlepage"><div><div>
<p class="title"><strong>Bouncing Results</strong></p>
</div></div></div>
<p>Imagine that you are sorting your results by a <code class="literal">timestamp</code> field, and
two documents have the same timestamp.  Because search requests are
round-robined between all available shard copies, these two documents may be
returned in one order when the request is served by the primary, and in
another order when served by the replica.</p>
<p>This is known as the <em>bouncing results</em> problem: every time the user refreshes
the page, the results appear in a different order. The problem can be avoided by always using the same shards for the same user,
which can be done by setting the <code class="literal">preference</code> parameter to an arbitrary string
like the user’s session ID.</p>
</div>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="_timeout_2"></a>timeout<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch-definitive-guide/edit/2.x/060_Distributed_Search/15_Search_options.asciidoc">edit</a>
</h3>
</div></div></div>
<p>By default, shards process all the data they have before returning a response to
the coordinating node, which will in turn merge these responses to build the
final response.</p>
<p>This means that the time it takes to run a search request is the sum of the time
it takes to process the slowest shard and the time it takes to merge responses.
If one node is having trouble, it could slow down the response to all search
requests.</p>
<p>The <code class="literal">timeout</code> parameter tells shards how long they
are allowed to process data before returning a response to the coordinating
node. If there was not enough time to process all data, results for this shard
will be partial, even possibly empty.</p>
<p>The response to a search request will indicate whether any shards returned a
partial response with the <code class="literal">timed_out</code> property:</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">    ...
    "timed_out":     true,  <a id="CO31-1"></a><i class="conum" data-value="1"></i>
    ...</pre>
</div>
<div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td align="left" valign="top" width="5%">
<p><a href="#CO31-1"><i class="conum" data-value="1"></i></a></p>
</td>
<td align="left" valign="top">
<p>The search request timed out.</p>
</td>
</tr>
</table>
</div>
<div class="warning admon">
<div class="icon"></div>
<div class="admon_content">
<p>It’s important to know that the timeout is still a best-effort operation; it’s
possible for the query to surpass the allotted timeout.  There are two reasons for
this behavior:</p>
<div class="olist orderedlist">
<ol class="orderedlist">
<li class="listitem">
Timeout checks are performed on a per-document basis.  However, some query types
have a significant amount of work that must be performed <span class="strong strong"><strong>before</strong></span> documents are evaluated.
This "setup" phase does not consult the timeout, and so very long setup times can cause
the overall latency to shoot past the timeout.
</li>
<li class="listitem">
Because the time is once per document, a very long query can execute on a single
document and it won’t timeout until the next document is evaluated.  This also means
poorly written scripts (e.g. ones with infinite loops) will be allowed to execute
forever.
</li>
</ol>
</div>
</div>
</div>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="search-routing"></a>routing<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch-definitive-guide/edit/2.x/060_Distributed_Search/15_Search_options.asciidoc">edit</a>
</h3>
</div></div></div>
<p>In <a class="xref" href="routing-value.html" title="Routing a Document to a Shard">Routing a Document to a Shard</a>, we explained how a custom <code class="literal">routing</code> parameter could be
provided at index time to ensure that all related documents, such as the
documents belonging to a single user, are stored on a single shard.  At search
time, instead of searching on all the shards of an index, you can specify
one or more <code class="literal">routing</code> values to limit the search to just those shards:</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">GET /_search?routing=user_1,user2</pre>
</div>
<p>This technique comes in handy when designing very large search systems, and we
discuss it in detail in <a class="xref" href="scale.html" title="Designing for Scale"><em>Designing for Scale</em></a>.</p>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="search-type"></a>search_type<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch-definitive-guide/edit/2.x/060_Distributed_Search/15_Search_options.asciidoc">edit</a>
</h3>
</div></div></div>
<p>The default search type is <code class="literal">query_then_fetch</code> . In some cases, you might want to explicitly set the <code class="literal">search_type</code>
to <code class="literal">dfs_query_then_fetch</code> to improve the accuracy of relevance scoring:</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">GET /_search?search_type=dfs_query_then_fetch</pre>
</div>
<p>The <code class="literal">dfs_query_then_fetch</code> search type has a prequery phase that fetches the term
frequencies from all involved shards to calculate global term
frequencies. We discuss this further in <a class="xref" href="relevance-is-broken.html" title="Relevance Is Broken!">Relevance Is Broken!</a>.</p>
</div>

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